PROJECT SUMMARY Neuropeptides play important roles in modulating neural circuits that process olfactory and gustatory infor- mation. This modulation generally functions to align an animal's internal state (e.g., levels of arousal, or food status) with behavioral responses to sensory stimuli (e.g., pheromones or food-associated odorants). Although the neuropeptidergic modulation of specific sensory neurons has been described, the molecular and cellular mechanisms by which neuropeptides affect central chemosensory circuits are unknown. The long-term goal is to characterize the neuropeptidergic modulation of chemosensory circuits by deconstructing this physiological process into clearly defined, behaviorally relevant molecular and neuronal events. Importantly, altered chemosensation in humans is often associated with certain types of mental disorders, and characterizing at a molecular level the neuropeptidergic modulation of chemosensation is a fundamental first step toward under- standing and improving treatments for these disorders. The Drosophila model system offers an excellent plat- form for these studies, because neuropeptidergic systems can be precisely manipulated and the effects on well-characterized circuits mediating chemosensation-induced behaviors, such as male aggressive behavior, can be measured. Initial experiments will focus on the neuropeptide tachykinin, with the central hypothesis that neuropeptides act locally to affect a specific population of neurons to modulate chemosensory information rele- vant to male agonistic behavior. The three specific aims, each based on preliminary success, are to: 1) charac- terize the neuropeptidergic circuit that modulates pheromone-induced male agonistic behavior, 2) characterize synergies between neuropeptides and co-released neurotransmitters, and 3) elucidate the cellular basis of in- teractions among three neuropeptide (tachykinin, neuropeptide F and FMRFamide) that each modulate male agonistic behavior (a chemosensation-guided behavior). In Aim 1, neurons expressing the tachykinin receptor (Takr86C) will be morphologically characterized, and those receiving synaptic input from aggression-promoting tachykininergic neurons will be identified via photo-activatable GFP-assisted neuronal tracing and in vivo cal- cium imaging. In Aim 2, interactions between tachykinin and the co-expressed neurotransmitter acetylcholine will be characterized. Peptide or transmitter release will be independently blocked in relevant cells, and effects on downstream neuronal activity and behavior will be analyzed. In Aim 3, anatomical and physiological rela- tionships between neurons expressing tachykinin, neuropeptide F, or FMRFamide will be established, leading to a better understanding of how these three neuropeptides synergistically affect a pheromone-processing neu- ral circuit. Together, the research proposed here will uncover the genetic and cellular mechanisms by which neuropeptides physiologically affect chemosensory circuits to modulate behavior. Such knowledge will provide a fundamental understanding of how smell and taste perception is centrally controlled, and how dysfunction of this process may lead to non-adaptive responses to environmental cues.